Tagging for the renderer
The phrase "Tagging for the renderer" and especially "Don't tag for the renderer" has a long history within OSM. More broadly it is sometimes phrased as "Don't map for the renderer" since it's not just about tagging. In any case the phrase is often misunderstood. The meaning of it is probably closer to the following:
- Don't deliberately enter data incorrectly for the renderer
but we need to live with the phrase that gets used most often. The basic good practice principle is that you avoid using incorrect tags, or otherwise skewing the data you enter, to make things show up in a specific way on the map rendering.
For example, if landuse=industrial shows up as a pink area on one of maps, and you have a flowerbed full of pink roses, then tagging your flowerbed as landuse=industrial would be incorrect and must be avoided. Instead, you should accurately tag the flowerbed with the type of plants, and improve the renderers instead so that they understand how to show it.
Another example is when someone abbreviates names, or puts spaces into them like T o w n H a l l S t r e e t to ensure that the names show up "nice" on the one particular map. This should be avoided as it is tagging for the renderer. It breaks the search, other map styles, and, in addition, map prettiness is subjective. It should be the renderers' job to abbreviate street names or interspace characters.
Under the impression that every rule needs breaking occasionally, Blackadder famously threw all standards out of the window to make a map of CERN by tagging the particle accelerator rings as highway=trunk and highway=primary (with tunnel=yes) even though they aren't major roads of any kind - he simply liked the colours and knew that they would show up. Don't follow his example! (Update: it has since been retagged to highway=path/tunnel=yes/access=restricted.)
To collect information about how buildings look use 3D tagging rather than map fake buildings, barriers and landuses that in some renderers will look like 3D image of building.
The Great Lakes were tagged as natural=coastline, even though that is for the ocean, because the coastline ways are rendered at low zoom levels. They were changed to water=lake and as a result disappeared from the default map layer. The correct fix is to teach the default stylesheet how to render lakes at low zoom levels.
Also, although the most common form of data skewing would be misuse of tags, this rule can apply to the manner in which elements are geometrically arranged.
A misunderstanding comes when people say that you shouldn't tag something "for the renderer" even though the tags being used are accurate and not misleading. For example, if a specialist map renders a particular specialist tag (e.g. the nesting site of rare birds) then using the tags the renderer understands is a perfectly reasonable thing to do, even if they haven't been formally "approved".